1. Field of the Invention
This invention relates to digital communications, and more specifically to the enhancement of speech quality when a bit stream representing a speech signal is subject to bit errors.
2. Related Art
In speech coding, sometimes called voice compression, a coder encodes an input speech or audio signal into a digital bit stream for transmission or storage. A decoder decodes the bit -stream into an output speech signal. The combination of the coder and the decoder is called a codec. When a transmission channel or storage medium introduces bit errors to the bit stream received by the decoder, the decoded speech will be degraded compared with the speech decoded from an error-free bit stream.
There are various ways for addressing bit errors. One well-known approach is forward error control, where a transmitter transmits additional redundant bits in such a way that allows a corresponding receiver to detect or even correct certain bit errors. With enough redundant bits transmitted, this scheme can be made very robust to bit errors. However, a drawback is that the overall bit rate is significantly increased by the transmission of redundant bits.
A second approach is to optimize quantizers within the codec for noisy channels with bit errors. Given a target bit error rate, in the design phase, the codec quantizers can be optimized to give a decoded signal that is better in the mean-squared error (MSE) sense, without the need to transmit redundant bits for forward error control. The problem is that such noisy-channel-optimized quantizers often have dynamic ranges smaller than the dynamic ranges of quantizers optimized for clear channel (i.e. channel free of bit errors). As a result, although the codec's performance is improved for the target bit error rate, for clear channel there is often a noticeable degradation in performance.
What is needed therefore is a technique for concealing bit errors in signals that exploits the natural redundancy in the transmitted signal parameters but does not degrade codec performance.